At present, most heating apparatuses supply a single type heat source to heat the tanks. Popular configurations include disposing devices such as heat sources, quartz tubes or electrical heating plate below the heating tanks. By burning fuels directly or converting electrical energy to thermal energy, the heat is transferred indirectly via the tank bodies of the heating tanks to the materials inside.
In this heating mode, the material closest to the bottom of the heating tank receives the thermal energy first and the temperature is gradually increased earlier than others. If the materials are in liquid state during the heating process, it is possible to transfer the heat through convection, thus the temperature of the overall materials can be increased more uniformly.
Nonetheless, liquid material with a high viscosity would hinder convection in the heating process. Consequently, the thermal energy provided by the heat sources may concentrate excessively in the region close to the bottom of the heating tanks and lead to nonuniform heating. Some materials may deteriorate due to overheating caused by heat retention.
When the heated materials are granular materials the contact areas among granules are small and nonuniform, resulting in increased thermal resistance (R), which becomes a great obstacle for heat transfer. The thermal resistance is defined as ΔT/q (° C./W or K/W), where ΔT(=Ti−To) is the temperature difference between two contact surfaces and q is the thermal transfer energy. Here, the thermal resistance Rt of the whole system includes the conduction part Rcd and the convection one Rcv. The conduction thermal resistance Rcd represents the resistant effect when heat is transferred by conduction. Taking heat transfer through filter granules as an example, the conduction thermal resistance is defined as Δx/(kA), where Δx is the thickness or distance of the thermal conductor, k is the thermal conductivity, and A is the thermal conduction area in-between. In addition to the interfaces of real contacts between granules, there are gaps without contacts, where gas flows through and results in extra thermal resistance Rt. The thermal resistance Rt, defined as 1/(hA), is caused by convection between the solid surfaces and fluids, where h is the heat transfer coefficient and A is the heat transfer surface area.
The conduction thermal resistance Rcd and the convection thermal resistance Rcv mentioned above cause obstacles to heat transfer; hence, the influence of the thermal resistances should be eased off in order to improve the heating efficiency. Possible options include improving the structural design of the heating tanks, stirring the granular materials by external work, leading in external hot gas, or changing the form of heat sources.
A method for solving the problems described above is to stir the granular materials by external force. This stirring action is to move the heated granules that are closer to the heat sources to the region with lower temperature, which does not rely on the existing heat transfer paths only. In addition, through stirring the granules with higher temperature can contact those not nearby initially, and thus shorten the heat transfer paths. In other words, the stirring action can mainly reduce the overall system conduction thermal resistance Rcd. During the stirring process, the gas flow among the granules can be driven and thereby slightly reduces the convection thermal resistance Rcv to improve the uniformity of the overall heat transfer. Nonetheless, in practice it is not easy to heat the granular materials close to the top as uniformly as those close to the bottom by simply stirring. Only the spin motion of the whole heating tank can provide sufficient stir. Unfortunately, spin motion is not a commonly available system, and is therefore not applicable to most cases; moreover, its installation and operation costs will raise financial barriers.
Another method is to change the type of the heat sources. For example, the heat source can be made in the type of serpentine tubes, which thus improves the range and region of heat supply. Nonetheless, in industrial heating tanks, even if the serpentine quartz heating tubes are adopted or the tubes filled with high-temperature liquid or gas are being used in the inner walls of said heating tanks, there is still room for improvement for supplying heat to the central part of the heating tanks.
Taiwan Patent Publication Number TW M302002 disclosed a baking apparatus combining two heat sources for baking materials. The appearance of the apparatus is a kiln. Inside the apparatus, the hot gas is led in from the bottom, guided upwards, and passes through a vent for heating. Meanwhile, there are multiple heating platforms disposed therein and heated by electric heaters. Nonetheless, such apparatus combining dual heat sources is only applicable to place a plurality of standalone items. There is no contact between the standalone items for heat conduction. Instead, they are arranged on the electric heaters for heating and baking; hence, the application range is quite limited, and the inner space is not utilized effectively, where materials are not fully filled for uniform heating. Accordingly, a real hybrid heating apparatus still awaits new technology for implementation.
In order to solve current technology problems, the present invention proposes a novel design of structure and method. Considering that the thermal resistance of a system comprises the conduction thermal resistance Rcd and the convection thermal resistance Rcv, the structure of the heating tank is improved and changed, and so that different methods are used for reducing the obstacles in the heat transfer caused by said factors. For the part of the conduction thermal resistance Rcd, according to the present invention, multiple sets of heating bars are inserted into the heating tank concurrently for controlling their distribution and thus reducing the conduction thermal resistance Rcd by enabling the heat to be conducted uniformly in the heating tank. For the problem of the convection thermal resistance Rcv, pipes are disposed for leading hot gas having sufficient thermal energy into the tank for reducing the convection thermal resistance Rcv. By applying both simultaneously, the total thermal resistance Rt of the system is lowered and the thermal efficiency is enhanced. Accordingly, the long-term problem of operation in the heating process of the industry is solved.